T cells play a central role in virtually all types of immune responses. These cells trigger antibody production, initiate delayed-type hypersensitivity, generate killer cells, and in some instances exert immunosuppressive effects. Our recent studies represent the first direct demonstration that these responses are not mediated by a single class of pluripotent T cells. Rather, this class is divisable into several subclasses, each of which is equipped to mediate a limited range of immune functions, each distinguishable by expression of different surface Ly markers. Thus cells capable of generating killer, helper and suppressor functions are likely to fall into distinct Ly positive subclasses. These findings open a new area of research which has important theoretical and practical implications. For example, studies of the relationships among the Ly positive T cell subclasses will yield new information concerning the differentiative process resulting in the formation of immunocompetent T cells. Insight provided from these studies into the patterning of normal T cell differentiation will provide a basis for investigation of the clinical syndromes arising from lesions at different points in the developmental pathway of T cells, resulting in different forms of autoimmune disease, thymic deficiency syndromes and oncogenesis. BIBLIOGRAPHIC REFERENCES: Huber, B., Gershon, R.K. and Cantor, H.: Identification of a B-cell surface structure involved in antigen-dependent triggering: absence of this structure on B cells from CBA/N mutant mice. J. Exp. Med. 145:10, 1977. Glimcher, L., Shen, F.W. and Cantor, H.: Identification of a cell-surface antigen selectivity expressed on the natural killer cell. J. Exp. Med. 145:1, 1977.